Ratio PV/RT dieterici’s equation

Hello1. The problem statement, all variables and given/known data
Determine the ratio PV/RT at the critical point for a fluid which obeys Dietericis equation
p(v-b)=RT exp(-a/RTv)

2. Relevant equations

3. The attempt at a solution
First ofall i don’t really understand this notion of critical point.
I found an equation to find it that is
(dp/dv)=(d²p/dv²) at constant temperature T
I don’t have any equation in my book so where did this get from ?
Using it I found that
V=2nb
P=a/(4b²e²)
T=a/4Rb
And then I don’t know what to do